Acute vascular diseases, such as myocardial infarction, stroke, pulmonary embolism, deep vein thrombosis, peripheral arterial occlusion, and other blood system thromboses constitute major health risks. Such diseases are caused by either partial or total occlusion of a blood vessel by a blood clot, which contains fibrin and platelets.
Current methods for the treatment and prophylaxis of thrombotic diseases involve therapeutics which act in one of two different ways. The first type of therapeutic inhibits thrombin activity or thrombin formation, thus preventing clot formation. These drugs also inhibit platelet activation and aggregation. The second category of therapeutic accelerates thrombolysis and dissolves the blood clot, thereby removing it from the blood vessel and unblocking the flow of blood [J. P. Cazenave et al., Agents Action. 15, Suppl., pp. 24-49 (1984)].
Heparin, a compound of the former class, has been widely used to treat conditions, such as venous thromboembolism, in which thrombin activity is responsible for the development or expansion of a thrombus. Although effective, heparin produces many undesirable side effects, including hemorrhaging and thrombocytopenia. This has led to a search for a more specific and less toxic anticoagulant.
Hirudin is a naturally occurring polypeptide which is produced by the blood sucking leech Hirudo medicinalis. This compound, which is synthesized in the salivary gland of the leech, is the most potent natural inhibitor of coagulation known. Hirudin prevents blood from coagulating by binding tightly to thrombin (K.sub.d =2.times.10.sup.-11 M) in a 1:1 stoichiometric complex [S. R. Stone and J. Hofsteenge, "Kinetics of the Inhibition of Thrombin by Hirudin", Biochemistry, 25, pp. 4622-28 (1986)]. This, in turn, inhibits thrombin from catalyzing the conversion of fibrinogen to fibrin (clot), as well as inhibiting all other thrombin-mediated processes [J. W. Fenton, II, "Regulation of Thrombin Generation and Functions", Semin. Thromb. Hemost., 14, pp. 234-40 (1988)].
The actual binding between hirudin and thrombin is a two-step process. Initially, hirudin binds to a "low" affinity site on the thrombin molecule (K.sub.d =1.times.10.sup.-8 M) which is separate from the catalytic site. This binding involves interaction of structure from the C-terminus of hirudin with an "anion-binding exosite" (ABE) in thrombin [J. W. Fenton, II et al., "Thrombin Anion Binding Exosite Interactions with Heparin and Various Polyanions", Ann. New York Acad. Sci., 556, pp. 158-65 (1989)]. Following the low affinity binding, the hirudin-thrombin complex undergoes a conformational change and hirudin then binds to the "high" affinity site on thrombin [S. Kono et al., "Analysis of Secondary Structure of Hirudin and the Conformational Change Upon Interaction with Thrombin", Arch. Biochem. Biophys., 267, pp. 158-66 (1988)]. This latter site corresponds to the active site of thrombin.
The isolation, purification and chemical composition of hirudin are known in the art [P. Walsmann and F. Markwardt, "Biochemical and Pharmacological Aspects of the Thrombin Inhibitor Hirudin", Pharmazie, 36, pp. 653-60 (1981)]. More recently, the complete amino acid sequence of the polypeptide has been elucidated [J. Dodt et al., "The Complete Covalent Structure of Hirudin: Localization of the Disulfide Bonds", Biol. Chem. Hoppe-Seyler. 366, pp. 379-85 (i985); S. J. T. Mao et al., "Rapid Purification and Revised Amino Terminal Sequence of Hirudin: A Specific Thrombin Inhibitor of the Blood-Sucking Leech", Anal. Biochem, 161, pp. 514-18 (1987); and R. P. Harvey et al., "Cloning and Expression of a cDNA Coding for the Anti-Coagulant Hirudin from the Bloodsucking Leech, Hirudo medicinalis", Proc. Natl. Acad. Sci. USA, 83, pp. 1084-88 (1986)].
At least ten different isomorphic forms of hirudin have been sequenced and have been shown to differ slightly in amino acid sequence [D. Tripier, "Hirudin: A Family of Iso-Proteins. Isolation and Sequence Determination of New Hirudins", Folia Haematol., 115, pp. 30-35 (1988)]. All forms of hirudin comprise a single polypeptide chain protein containing 65 or 66 amino acids in which the amino terminus primarily comprises hydrophobic amino acids and the carboxy terminus typically comprises polar amino acids. More specifically, all forms of hirudin are characterized by an N-terminal domain (residues 1-39) stabilized by three disulfide bridges in a 1-2, 3-5, and 4-6 half-cysteinyl pattern and a highly acidic C-terminal segment (residues 40-65). In addition, the C-terminal segment of hirudin is characterized by the presence of a tyrosine residue at amino acid position 63 which is sulfated
In animal studies, hirudin, purified from leeches, has demonstrated efficacy in preventing venous thrombosis, vascular shunt occlusion and thrombin-induced disseminated intravascular coagulation. In addition, hirudin exhibits low toxicity, little antigenicity and a very short clearance time from circulation [F. Markwardt et al., "Pharmacological Studies on the Antithrombotic Action of Hirudin in Experimental Animals", Thromb. Haemost., 47, pp. 226-29 (1982)].
In an effort to create a greater supply of hirudin, attempts have been made to produce the polypeptide through recombinant DNA techniques. The presence of an O-sulfated tyrosine residue on native hirudin and th inability of microorganisms to perform a similar protein modification made the prospect of recombinant production of biologically active hirudin highly speculative. The observation that desulfatohirudins were almost as active as their sulfated counterparts [U.S. Pat. No. 4,654,302], however, led the way to the cloning and expression of hirudin in E.coli [European patent applications 158,564, 168,342 and 171,024] and yeast [European patent application 200,655]. Despite these advances, hirudin is still moderately expensive to produce and it is not widely available commercially.
Recently, efforts have been made to identify peptide fragments of native hirudin which are also effective in prolonging clotting times. An unsulfated 21 amino acid C-terminal fragment of hirudin, N-acetylhirudin.sub.45-65, inhibits clot formation in vitro. In addition, several other smaller, unsulfated peptides corresponding to the C-terminal 11 or 12 amino acids of hirudin (residues 55-65 and 54-65) have also demonstrated efficacy in inhibiting clot formation in vitro [J. L. Krstenansky et al., "Antithrombin Properties of C-terminus of Hirudin Using Synthetic Unsulfated N-acetyl-hirudin.sub.45-65 ", FEBS Lett, 211, pp. 10-16 (1987)]. Such peptide fragments, however, may not be fully satisfactory to dissolve blood clots in on-going therapy regimens because of low activity. For example, N-acetyl-hirudin.sub.45-65 has a specific activity four orders of magnitude lower than native hirudin.
In addition to catalyzing the formation of a fibrin clot, thrombin has several other bioregulatory roles [J. W. Fenton, II, "Thrombin Bioregulatory Functions", Adv. Clin. Enzymol., 6, pp. 186-93 (1988)]. For example, thrombin directly activates platelet aggregation and release reactions. This means that thrombin plays a central role in acute platelet-dependent thrombosis [S. R. Hanson and L. A. Harker, "Interruption of Acute Platelet-Dependent Thrombosis by the Synthetic Antithrombin D-Phenylalanyl-L-Prolyl-L-Arginylchloromethylketone", Proc. Natl. Acad. Sci. USA, 85, pp. 3184-88 (1988)]. Thrombin can also directly activate an inflammatory response by stimulating the synthesis of platelet activating factor (PAF) in endothelial cells [S. Prescott et al., "Human Endothelial Cells in Culture Produce Platelet-Activating Factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) When Stimulated With Thrombin", Proc. Natl. Acad, Sci. USA, 81, pp. 3534-38 (1984)]. PAF is exposed on the surface of endothelial cells and serves as a ligand for neutrophil adhesion and subsequent degranulation [G. M. Vercolletti et al., "Platelet-Activating Factor Primes Neutrophil Responses to Agonists: Role in Promoting Neutrophil-Mediated Endothelial Damage", Blood, 71, pp. 1100-07 (1988)]. Alternatively, thrombin may promote inflammation by increasing vascular permeability which can lead to edema [P. J. Del Vecchio et al., "Endothelial Monolayer Permeability To Macromolecules", Fed. Proc., 46, pp. 2511-15 (1987)]. Reagents which block the active site of thrombin, such as hirudin, interrupt the activation of platelets and endothelial cells [C. L. Knupp, "Effect of Thrombin Inhibitors on Thrombin-Induced Release and Aggregation", Thrombosis Res., 49, pp. 23-36 (1988)].
Thrombin ha also been implicated in promoting cancer, based on the ability of its native digestion product, fibrin, to serve as a substrate for tumor growth [A. Falanga et al., "Isolation and Characterization of Cancer Procoagulant: A Cysteine Proteinase from Malignant Tissue", Biochemistry, 24, pp. 5558-67 (1985); S. G. Gordon et al., "Cysteine Proteinase Procoagulant From Amnion-Chorion", Blood, 66, pp. 1261-65 (1985); and A. Falanga et al., "A New Procoagulant in Acute Leukemia", Blood. 71, pp. 870-75 (1988)]. And thrombin has been implicated in neurodegenerative diseases based on its ability to cause neurite retraction [D. Gurwitz et al., "Thrombin Modulates and Reverses Neuroblastoma Neurite Outgrowth", Proc. Natl. Acad. Sci. USA, 85, pp. 3440-44 (1988)]. Therefore, the ability to regulate the in vivo activity of thrombin has many important clinical implications.
Despite the developments to date, the need still exists for a molecule that effectively inhibits thrombin function in clot formation, platelet activation and various other thrombin-mediated processes and which can be produced inexpensively and in commercially feasible quantities.